Related fields include transparent conductive thin films, particularly for use with optoelectronic devices including III-V materials, and physical vapor deposition (PVD), particularly sputtering.
A typical LED emits light from an active photoemissive semiconductor layer sandwiched between p-type and n-type semiconductor layers. Electroluminescence results when negative charge carriers (electrons) from the n-type layer and positive charge carriers (“holes”) from the p-type layer meet and combine in the active photoemissive layer.
The wavelength and color of the emitted light depends, at least in part, on the semiconductor bandgap. For example, arsenides of aluminum (Al), gallium (Ga), indium (In), and their alloys emit red and infrared light; phosphides of Al, Ga, In, and their alloys emit green, yellow, or red light; and nitrides of Al, Ga, In, and their alloys emit ultraviolet, violet, blue, or green light. These “III-V materials,” so-called because they include elements from old group III (now group 13) and old group V (now group 15) of the periodic table, have high carrier mobility and direct bandgaps that are desirable in optoelectronic applications. However, substrates made of III-V materials have historically been very expensive. GaN and AlN substrates are becoming increasingly available, but problems with instability and defects persist. A common alternative approach has been to grow III-V layers by epitaxy on a different substrate material such as sapphire (Al2O3), silicon (Si), silicon carbide (SiC), germanium (Ge), zinc oxide (ZnO), or glass.
A “junction-up” LED emits its output light in a direction pointing away from the substrate, while an inverted, or “flip-chip,” LED emits its output light toward the substrate. Both types may use transparent electrodes on the light-emitting side to facilitate the passage of both electrical current and light through the semiconductor stack. Other devices with similar requirements for current and light traversing the same surface also use transparent electrodes. Many thin-film materials for transparent electrodes are oxides, and are generically known as “transparent conductive oxides” (TCO).
Indium tin oxide, (ITO), the most common TCO material for transparent electrodes, is expensive because it is typically over 90% indium (In). The optical transparency and the conductivity generally need to be traded off against each other because the highest-transparency formulations are generally different from the highest-conductivity formulations. In addition, both the optical transparency and the conductivity may be unstable with temperature, and therefore may change unpredictably during high-temperature process steps, such as annealing, that may be required to fabricate either the TCO itself or other parts of the device.
As with any electrode material, another variable that needs to be optimized is the work function. If a low-loss ohmic contact to a semiconductor is desired, a generally necessary (though not always sufficient) condition is a close match between the electrode work function and the semiconductor work function. High-indium ITO is one of a few TCO materials that can be tuned to match the work function of p-GaN.
Therefore, a need exists for a cost-effective TCO material with a work-function matching that of p-GaN (4.2 eV or greater), exhibiting low absorption coefficient (<0.03%/nm) and low resistivity (contact resistance<0.005 Ω-cm2) throughout the temperature range of fabrication processes for LEDs and other optoelectronic devices. Smooth morphology and compatibility with other processes used in making the device are also desirable.